As shown in JP-2000-170588 A, there is proposed an engine control system which includes a supercharging pressure control unit for performing a supercharging pressure control by controlling a variable blade (variable nozzle) of a variable nozzle turbocharger, and an EGR control unit for performing an exhaust gas recirculation volume control (EGR control) by controlling an exhaust gas recirculation volume control valve (EGR control valve) of an exhaust gas recirculation device. Incidentally, the supercharging pressure control is constructed so as to perform feedback control of the opening degree of the variable blade on the basis of a deviation between an actual supercharging pressure and a target supercharging pressure so that the actual supercharging pressure (actual intake pressure detected by an intake pressure sensor) substantially coincides with a target supercharging pressure (target intake pressure) set according to the operation state of an engine. The exhaust gas recirculation volume control is constructed so as to perform feedback control of the opening degree of the EGR control valve on the basis of a deviation between an actual fresh intake volume and a target fresh intake volume so that the actual fresh intake volume detected by an intake volume sensor substantially coincides with the target fresh intake volume set according to the operation state of the engine.
However, in the conventional engine control system, when the EGR control valve is opened, as shown in FIG. 1, pressure of an “A” area of an exhaust path from an exhaust port of the engine to the variable nozzle of the variable nozzle turbocharger and an exhaust gas recirculation passage at the exhaust path side of the EGR control valve is lowered. The flow velocity of the exhaust gas blown into a turbine wheel from the exhaust path of the engine is decreased, and the rotation of the turbine wheel is decreased.
When the rotation of the turbine wheel is decreases, the rotation of a compressor wheel coaxial with the turbine wheel is also decreased and the actual supercharging pressure is decreased. In order to secure the target intake pressure, the variable nozzle is closed according to the deviation between the actual supercharging pressure and the target supercharging pressure.
When the variable nozzle is closed, the pressure of the “A” area is raised, the exhaust gas recirculation volume (EGR volume) of the exhaust gas recirculation path is increased by the rise and the actual fresh intake volume is decreased. Therefore, in order to secure the target fresh intake volume, the EGR control valve is closed according to the deviation between the actual fresh intake volume and the target fresh intake volume.
As described above, when the EGR control and the supercharging pressure control perform the feedback control independently of each other, there occurs such a problem that the control is not stabilized, and according to circumstances, the control command value of the variable nozzle and the control command value of the EGR control valve hunt (oscillate at a specified frequency).